In short, Microsoft is opting in all US advertisers into their content network (i.e. ContentAds). The only way for you to say you don't want to be included in the content network is to fill out a form prior or turn it off after you are opted into the network. More on that over here.

Publishers, listen up. This does not mean Microsoft is launching an AdSense competitor on the 29th. All they are doing is allowing all advertisers to put their ads on Microsoft's content network. The content network from the publisher side is currently locked down. So no, you can't put Microsoft ContentAds on your sites, just yet.